There are many applications in, for example, industry, science and medicine where a high voltage power supply capable of generating over 200 kilovolts at a current in the milliamp range, for example, 5 ma (i.e., over one kilowatt of output power) are required. While a number of high voltage power supply designs are available, a multistage cascade rectifier power supply is the preferred design for many such applications, offering the best cost-performance ratios.
In a multistage cascade rectifier power supply, each capacitor rectifier stage has a more or less predetermined voltage gradient and a desired output voltage is obtained by utilizing the number of such stages which is required in order to achieve the desired output level. However, voltage droop in such a power supply (i.e., the DC voltage drop with load) increases as the cube of the number of stages and the voltage ripple, the fluctuation in the output voltage with load (i.e., the AC component o the high voltage output) increases linearly with the number of stages. The component and assembly costs of the power supply also increase with the number of stages.
It is therefore desirable that the voltage gradient per stage be as high as possible so as to minimize the number of stages required to achieve a given voltage output. There are, however, several problems in designing a high gradient cascade rectifier power supply. First, the electrical field stresses produced for each stage must be maintained below the breakdown value of the insulating gas, the capacitors, and any other materials which form part of the stages so as to avoid any breakdown in a given stage. One problem in assuring that breakdown does not occur is that, even though the average potential gradient of a stage may be well below the component breakdown values, nonuniformities may occur in the field which can result in local "hot spots" where breakdown may occur.
A second factor is that a high gradient cascade rectifier design is characterized by the inequality: ##EQU1## where: V.sub.i is the RMS input drive voltage to the rectifier, f is the driving frequency, P.sub.o is the DC power delivered to the load and C.sub.s is the stray shunt capacitance of the power supply at the input drive terminals. When the above inequality is satisfied, the shunt reactive input impedance is smaller than the shunt resistive input impedance, the shunt reactive input impedance resulting primarily from the stray capacitance of the cascade rectifier structure. There are a number of advantages in designing a cascade rectifier power supply which satisfies the above inequality. When compared to cascade rectifier designs which do not satisfy this inequality, such designs typically offer more voltage per inch and thus more voltage per stage, operate at higher frequency, and, requiring fewer stages, minimize voltage droop and ripple. However, in practice it is difficult to design efficient cascade rectifiers which satisfy this inequality. In particular, in order for such a rectifier design to be practical, the stray capacitance must be "tuned out" to a sufficient degree at the driving frequency.